WDXS31 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 18S (NEVILLE) WARNING 
NR 035//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8S 105.7E
   INITIAL INTENSITY: 85 KTS
   GEOGRAPHIC REFERENCE: 545 NM WEST-NORTHWEST OF LEARMONTH,
AUSTRALIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 27 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY DEPICTS TROPICAL CYCLONE
(TC) 18S WITH PERSISTENT DEEP CONVECTION OVER THE CENTER ALONG WITH
A DIMPLE-EYE FEATURE AND WELL-DEFINED CONVECTIVE BANDING WRAPPING
INTO THE SYSTEM. THE CURRENT ENVIRONMENT IS ASSESSED AS FAVORABLE
CHARACTERIZED BY MODERATE RADIAL OUTFLOW ALOFT, LOW (0-5 KTS)
VERTICAL WIND SHEAR (VWS), AND WARM (27-28 C) SEA SURFACE
TEMPERATURES. MODELS SUGGEST THAT THE CORE OF THE VORTEX HAS
REMAINED MOIST AND IS SUCCESSFULLY RESISTING DRY AIR ENTRAINMENT.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A
0600.HM9.AHI.VIS1KM IMAGE REVEALING A FORMATIVE EYE. THE INITIAL
INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
SUBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) TO THE SOUTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   KNES: T5.5 - 102 KTS
   DEMS: T4.5 - 77 KTS
   APRF: T4.5 - 77 KTS
   CIMSS DPRINT: 84 KTS AT 210700Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 0-5 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE RADIAL

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: LOW

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TC 18S IS FORECAST TO CONTINUE ON A WESTWARD
TRACK UNTIL TAU 24 AS IT TRACKS ALONG THE NORTHERN PERIPHERY OF THE
STR TO THE SOUTH. AFTER TAU 24, THE SYSTEM IS FORECAST TO BEGIN A
SOUTHWESTWARD TRACK AS IT STARTS TO ROUND THE NORTHWESTERN
PERIPHERY OF THE STR. THE SYSTEM IS EXPECTED TO CONTINUE THE
SOUTHWESTWARD TRACK THROUGH THE REMAINDER OF THE FORECAST INTERVAL.
REGARDING THE JTWC INTENSITY FORECAST, TC 18S IS FORECAST TO
INTENSIFY TO A PEAK OF 95 KTS AROUND TAU 12 AND MAINTAIN THAT
INTENSITY THROUGH TAU 24 DUE PRIMARILY TO AN INCREASE IN OUTFLOW
ALOFT AND THE PERSISTENCE OF LOW VWS. AFTER TAU 24, VWS VALUES
BEGIN TO RISE TO AROUND 15 KTS AND THE SYSTEM IS EXPECTED TO START
DEGRADING. TC 18S IS, AS A RESULT, FORECAST TO BE 90 KTS NEAR TAU
36 AND 85 KTS NEAR TAU 48. AFTER TAU 48, DRY AIR BEGINS TO ENTRAIN
INTO THE SYSTEM AND VWS IS EXPECTED TO INCREASE TO 25-30 KTS DUE TO
PROXIMITY TO THE UPPER-LEVEL JET WHICH WILL AID TO QUICKLY WEAKEN
THE SYSTEM. TC 18S IS FORECAST TO WEAKEN TO 60 KTS NEAR TAU 72 AND
EVENTUALLY DISSIPATE TO 30 KTS NEAR TAU 96.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72 WITH A 250 NM CROSS-TRACK SPREAD AT TAU 72. GUIDANCE
BEGINS TO SPREAD AFTERWARDS AND THE CROSS-TRACK SPREAD INCREASES TO
350 NM AT TAU 96 AS THE MODELS VARY ON THE SHARPNESS OF THE TURN
AROUND THE STR. THEREFORE, THE JTWC TRACK FORECAST IS ASSESSED WITH
HIGH CONFIDENCE THROUGH TAU 72 AND MEDIUM CONFIDENCE FOR THE
REMAINDER OF THE FORECAST. THE JTWC INTENSITY FORECAST IS ASSESSED
WITH OVERALL LOW CONFIDENCE DUE TO TECHNICAL DIFFICULTIES
PREVENTING GFS, HWRF, AND HAFS-A FROM BEING PRESENT IN THE MODEL
GUIDANCE FOR THIS WARNING.  

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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